In the recent years, electronic instruments have been remarkably made thinner and more compact, and their reliability has been greatly enhanced with the increase in the density of integration and their application has steadily expanded. A monolithic IC has undergone a rapid increase in the density and compactification, while also in the hybrid IC field, especially in industrial instruments such as control circuits for automobiles, power module devices etc., there has been made an effort toward large-sized hybrid IC having excellent heat resistance and thermal shock resistance. In the latest hybrid IC a ceramic substrate carries active components such as diodes, transistors, semiconductor IC etc. and also almost all electrical parts such as coils, transformers, condensers etc. Hybrid integrated circuits having a further increased degree of integration and greatly enhanced reliability have been developed.
The hybrid IC is constructed by packaging individual parts or IC elements on a ceramic substrate on by applying thick film-forming techniques. A cerdip IC is generally made by fixing silicon IC chips on an alumina substrate comprising 91-96% Al.sub.2 O.sub.3 using a bonding paste, but it still needs even higher strength bond imparting better durability.
In general, for the bonding method for cerdip, Au based pastes, solder, glass etc. have been used, as described in, for example, U.S. Pat. No. 3,846,345. The Au based pastes have excellent electrical conductivity, are chemically stable, have the best bondability with Au wires, can easily be alloyed with Si, and are extremely good in bonding with substrate as described in U.S. Pat. No. 3,799,891. However, they have a disadvantage of being expensive. In order to eliminate this disadvantage, Ag--Pd based pastes have been developed which contains Ag instead of Au and also contained Pd so as to prevent the migration of Ag, as described in U.S. Pat. Nos. 3,929,491 and 3,918,980.
These conventional pastes are those obtained by mixing metal powders with vitreous metal oxides and kneading using vehicles, and in bonding with alumina substrate they rely solely on the bonding force due to sintering of the glass frit.
However, the glass frit is poor in thermal shock resistance and has a disadvantage that the bond strength is thermally deteriorated in a step of sintering substrate and packaging or by the change in surrounding temperature on use. Although attempts have been made to chemically combine it with alumina substrate by adding very small amounts of Cu etc. in order to enhance the bonding strength with the alumina substrate (See U.S. Pat. Nos. 3,929,491 and 3,918,980), it is difficult to greatly improve the heat deterioration characteristics as long as the glass frit is used. In other words, the mere addition of Cu fine powder causes separation of the Cu from other metal fine powders in the vehicle due to the difference in specific gravity, resulting in poor distribution on dotting, and therefore not only failing to give a uniform paste film but also giving a film having inadequate bonding strength because of poor diffusion on the alumina substrate. Further, the part where Cu is segregated during the course of firing undergoes localized oxidation and gets colored, and therefore a film having a uniform smooth surface cannot be obtained.